This invention relates to optical stereolithography processes for rapid prototyping. More particularly, it relates to a method for determining the areas of resin to be cured in an optical stereolithography process for rapid prototyping. This invention also provides a method for controlling an optical stereolithography system to reduce the inaccuracy or oversizing of parts in the part built (z-) direction due to overpenetration of the laser beam and overcuring of the resin.
Optical stereolithography is a manufacturing technology that uses a sequential additive layer method to build complex three dimensional objects from computer data. The technique uses a combination of computer graphics, laser polymerization and photochemistry technologies. Its many benefits include significant productivity gains and cost saving in product design, and the ability to introduce concept models in a short period of time. The process relies on a scanning ultraviolet laser beam to harden successive thin layers of photopolymer, building each layer on top of the previous layer until a three-dimensional part has been formed.
The technology is popular in industry for rapid prototyping. Considerable effort has been directed to improving the accuracy of the fabricated model, especially the accuracy in the vertical or Z-direction by reducing oversizing. The Z-directional inaccuracy or oversizing can be as high as 200 xcexcm compared to 50 xcexcm in the X and Y directions. The X- and Y-errors, however, can be corrected easily in most optical stereolithography machines. In spite of its great potential, the dimensional errors, if not corrected properly, will limit the application of rapid prototyping parts to design verification only.
There are two causes of the oversizing. The first is that the laser power is normally selected to cure the photomonomer to a depth, d greater than the layer thickness, Lp to ensure bonding between the layers. Thus, when curing the bottommost layer of an over-hanging part, below which is liquid resin rather than the stage or the previously cured layer, a layer is formed thicker than the desired layer pitch.
The second cause is that when the second layer is scanned or cured by the laser beam, the liquid resin below the previously cured layer is also cured. This is due to the over-penetration effect of the laser, suggesting that the solid resin is more transparent to the UV laser than the liquid resin. This over penetration effect may give rise to further overcuring of the liquid resin below the cured layers when a third layer is scanned by the laser. FIG. 1 shows the oversizing effect caused by over-penetration of the laser. The following is shown in FIG. 1:
Lp: desired layer pitch (also the amount the stage will be lowered after each step).
d1: cure depth of the first layer.
d2: total cure depth of the first and second layers.
d3: total cure depth of the first, second and third layers.
xcex4d: oversizing due to over-penetration effect of the laser beam.
FIG. 1 compares the actual depth, d, and the desired layer pitch, Lp, of the cured layer 1 for both a part overhanging a forming stage 2 and a non-overhanging and an overhanging part. It also shows the overcuring 3 from a single scan of layer 1 and the oversizing effect 4, 5 caused by over-penetration of the laser from scanning a second layer 6 and a third layer 7.
More recent US patents on optical stereolithography, such as U.S. Pat. No. 5,256,340, and U.S. Pat. No. 5,571,471, offer no solution to overcome the inaccuracy due to the oversizing effect. U.S. Pat. No. 5,398,193 teaches a subtractive or removal procedure to remove surplus material formed in various forming processes. Japanese patent applications No. 125078:1995 and No. 125079:1995 teach off-setting the 3-D model data with the value of the oversize to improve the accuracy of the model.
An object of this invention is to provide a method for determining the areas of resin to be cured in an optical stereolithography process for rapid prototyping. It is a further object of the invention to provide an improved optical stereolithography process to produce more accurate models for rapid prototyping.
In one aspect this invention provides a method for determining the areas of resin to be cured in an optical stereolithography process for rapid prototyping using sequential laser curing of layers of a resin having a selected thickness to produce a desired shape of cured resin, said method comprising the steps of:
(a) generating a computer model of said desired shape;
(b) dividing the model into layers of thickness equal to the thickness of said layers each defining an area of resin to be cured by corresponding layer data;
(c) selecting a set of said layers including and sequentially adjacent an intermediate layer and determining the areas in said intermediate layer of resin which would be oversized by said laser curing of one or more layers later in the curing sequence;
(d) modifying said layer data to compensate for said oversizing; and
(e) repeating steps (c) and (d) by sequentially selecting said sets of layers to include substantially all of said layers respectively as said intermediate layers.
Preferably the sets are sequentially selected with each subsequent set having the intermediate layer being the layer adjacent the intermediate layer of the previously selected set.
It is also preferred that the first selected set includes the first layer to be cured and the sequential subsequent sets respectively include as the intermediate layer the subsequently cured adjacent layer.
Preferably each set includes N layers where N is selected from the series 3, 5, 7 or higher odd numbers determined by the size of possible oversizing from laser curing of the resin in terms of numbers of layer thickness to be compensated.
In one specific form the invention provides a method to compensate for oversizing of two layer thickness including:
(a) naming polygons defined by said layers in the computer model as P1, P2 . . . PK . . . starting at the first layer to be cured;
(b) selecting the first five layers P1 to P5 as said selected set and determining said intermediate layer to be middle layer P3;
(c) determining the areas in layer P3 that are potentially oversized by one layer thickness due to laser curing of layer P4, denoted by Poly 1=P3∩P4;
(d) determining the areas in Poly 1 in which oversizing would actually occur as a result of no cured resin in corresponding areas of the adjacent earlier cured layer, by finding the difference between Poly 1 and P2, denoted by Poly 2=Poly 1∩{overscore (P2)}.
(e) determining the areas in P3 that are potentially oversized due to laser curing of layer P5, denoted by Poly 3=Poly 1∩P5xe2x89xa1P3∩P4∩P5;
(f) determining the areas in Poly 3 in which a second layer oversizing will actually occur as a result of no cured resin in corresponding areas two layers earlier by finding the difference between Poly 3 and P1, denoting the area as Poly 4=Poly 3∩{overscore (P1)}; and
(g) sequentially repeating the steps (b) to (f) for each set of 5 layers starting from the second layer of the previous set and continuing until all sets of layers in the model have been processed.
Preferably, the computer model is modified by removing from layer P3 all the areas that would produce oversizing one and two layers below it and denoting the resultant polygon, represented by P3∩{overscore (Poly 2)}∩{overscore (Poly 4)}, as the modified layer.
In another specification the invention provides a method to compensate for oversizing of two layer thickness including
(a) naming polygons defined by said layers in the computer model as P1, P2 . . . PK . . . starting at the first layer to be cured;
(b) selecting the first five layers P1 to P5 as said selected set and determining said intermediate layer to be middle layer P3;
(c) determining the areas in layer P3 that are potentially oversized by one layer thickness due to laser curing of layer P4, and representing the areas (P3∩P4)∩{overscore (P2)}xe2x89xa1P3∩P4∩{overscore (P2)};
(d) determining in layer P3 where oversize would occur two layers earlier as a result of laser curing in layer P5, and representing the areas by P3∩P4∩P5∩{overscore (P1)}; and
(e) modifying the computer model by removing from layer P3 all the areas where oversize would occur below it as a result of laser curing in layers P4 and P5, the resultant polygon being represented by P3∩{overscore ([(P3∩P4)∩)}{double overscore (P2)}{overscore (])}∩{overscore ([P3∩P4∩P5])}∩{double overscore (P1)}xe2x89xa1P3∩({overscore (P3)}∩{overscore (P4)}∩P2)∩({overscore (P3)}∩{overscore (P4)}∩{overscore (P5)})∩P1
Preferably, sequential layers are cured in a z direction of an orthogonal x, y, z co-ordinate system. The z direction, preferably, corresponds to the vertical direction or built direction in the stereolithography process so that the first cured layer is the lower layer.
The modified layer data can be stored for example in a computer memory, magnetic media or optical media.
The invention also includes within its scope a method for controlling an optical stereolithography system for rapid prototyping to reduce oversizing including the above method of determining the areas of resin to be cured.
One embodiment of the invention will now be described by way of example only.